The initial observation of the photoinduced decomposition of water on titanium dioxide (TiO2) has promoted considerable interest in solar cells and the semiconductor-based photocatalytic decomposition of water and of other organic materials in polluted water and air. A continued focus on TiO2 has resulted because of its relatively high reactivity and chemical stability under ultraviolet excitation (wavelength<387 nanometers), where this energy exceeds the bandgaps of both anatase (3.2 eV) and rutile (3.0 eV) crystalline n-TiO2.
However, both anatase and rutile TiO2 crystals are poor absorbers in the visible region (wavelength>380 nm) and the cost and accessibility of ultraviolet photons make it desirable to develop photocatalysts that are highly reactive under visible light excitation, utilizing the solar spectrum or even interior room lighting.
With this focus, several attempts have been made to lower the bandgap energy of crystalline TiO2 by transition metal doping and hydrogen reduction. One approach has been to dope transition metals into TiO2 and another has been to form reduced TiOx photocatalysts. However, doped materials suffer from a thermal instability, an increase of carrier-recombination centers, or the requirement of an expensive ion-implantation facility. Reducing TiO2 introduces localized oxygen vacancy states below the conduction band minimum of titanium dioxide so that the energy levels of the optically excited electrons will be lower than the redox potential of the hydrogen evolution and the electron mobility in the bulk region will be small because of the localization.
Films and powders of titanium oxynitride (TiO2−xNx) have revealed an improvement over titanium dioxide under visible light in optical absorption and photocatalytic activity such as photodegradation of methylene blue and gaseous acetaldehyde, and hydrophilicity of the film surface. Substitutional doping of nitrogen by sputtering a titanium dioxide target in a nitrogen/argon gas mixture has been accomplished. After being annealed at 550° C. in nitrogen gas for four hours, the films were crystalline with features assignable to a mixed structure of the anatase and rutile crystalline phases. The films were yellowish in color and their optical absorption spectra showed them to absorb light between 400–500 nm, whereas films of pure titanium dioxide did not. Photocalytic activity for the decomposition of methylene blue shows activity of TiO2−xNx at wavelengths less than 500 nm.
The active wavelength of TiO2−xNx of less than 500 nm promises a wide range of applications, as it covers the main peak of the solar irradiation energy beyond Earth's atmosphere. Further, it is an excellent light source, peaking at 390 to 420 nm, provided by recently-developed light-emitting indium gallium nitride diodes.
In addition, nitrogen can be incorporated into the TiO2 structure by the nitridation reaction of TiO2 nanopowders that are subjected to a ammonia (NH3) gas flow at about 600° C. Transmission electron microscope micrographs showed that the synthesized TiN powder consisted of uniform spherical particles with an average diameter of about 20 nm when nitridation was performed at a temperature of about 600° C. for 2–5 hours. No results with respect to the photocatalytic activity of this material were presented.
The synthesis of chemically modified n-type TiO2 by the controlled combustion of Ti metal in a natural gas flame at a temperature of about 850° C. represented another attempt at lowering the band gap energy of TiO2. The modified films were dark gray, porous in structure and with an average composition of n-TiO2−xCx (with x about 0.15). This material absorbs light at wavelengths below 535 nm and has a lower band-gap energy than rutile TiO2 (2.32 versus 3.00 electron volts). When illuminated with a 150 Watt xenon (Xe) lamp, and at an applied potential of 0.3 volt, the chemically modified n-TiO2−xCx (with x about 0.15) exhibited a higher water photoconversion efficienty (8.3%) than that of pure TiO2 illuminated under the same conditions (1%).
All these examples require the use of very high temperature synthesis conditions, and long periods of time to produce these materials. The time and temperature previously required to make the TiO2−xNx and TiO2−xCx compounds makes these techniques costly and inefficient.
Thus, a heretofore unaddressed need exists in the industry for a simple more cost effective method to fabricate novel materials capable of exhibiting photo catalytic activity such as the photo-induced decomposition of water and pollutants. Additionally, a need exists for better methods for their use in the production of electricity through solar cells, as well as to address some of the aforementioned deficiencies and/or inadequacies.